The present invention relates to electrophotography and more particularly, to a magnetic brush developing apparatus for use in an electrophotographic copying machine of the toner powder image transfer type employing electrically conductive and magnetically attractive toner particles having high resistance.
Recently, intensive studies have been made of developing apparatus employing a mono-component developer developing method, since such one component toner is not subject to variation in the mixing ratio of the toner particles and carrier material as in a two component developer, with consequent facilitation of the maintenance of the copying machines.
While the developing method employing a mono-component developer as described above is effective for a direct type copying machine such as a so-called electrofax type, it causes various problems when applied to toner powder image transfer type copying machines. More specifically, in toner powder image transfer type copying machines, when the developing and transfer are effected with low resistance magnetically attractive toner used for a direct type copying machine, resultant images are affected by "dimming or hazing" i.e. indefinite contour at the time of transfer, although the developing is favorably carried out with high density. Such a disadvantage as described above is considered to be possibly attributable to the phenomenon that, because of the low resistance of the toner particles charge imparted to a transfer material or copy paper is poured into the toner particles, when the toner particles are transferred onto the copy paper, so that the copy paper and the toner particles are given the same polarity, with the result that the toner particles once transferred onto the copy paper are again repelled from said copy paper or repelled to the surrounding portion of the image formed on the copy paper. Therefore, such a mono-component developer for use in toner powder image transfer type copying machines should have a high resistance. However, if such high resistance magnetically attractive toner particles are employed in conventional magnetic brush developing apparatuses, there is an inconvenience that the density of the copied images tends to be low, since in such conventional magnetic brush developing apparatuses, the magnetic binding force acting on the toner particles is comparatively large.
In order to overcome the drawback as described above, there has heretofore been proposed, for example, in Japanese Laid Open Patent Application No. 51-105345, a developing apparatus in which the developing time is made comparatively long for obtaining copied images of high density, which developing apparatus, however, also has a disadvantage that undesirable fogging is produced in the non-image portion resulting in deterioration of the quality of copied images.